Electrical circuits, such as printed circuit boards (PCBs), integrated circuits, and flat-panel displays (FPDs), may be tested for defects using infrared (IR) thermography. In general, power is applied to a device under test (DUT) to heat various of the device features. An infrared detector then captures a test image of the heated DUT. The resulting image, a collection of pixel-intensity values spatially correlated to the imaged object, is then compared with a similar collection of reference image data. Differences between the test and reference data, typically stored as a “composite image,” indicate the presence of defects.
Defect identification algorithms analyze composite images to automatically identify defects, and consequently improve throughput and quality in device manufacturing. Examples of such inspection systems include, but are not limited to, inspection of FPDs, PCBs, MEMS-based devices, semiconductor devices, and biomedical specimen. One purpose of such systems is to test for defects potentially present on a device at some critical point during manufacture of that device. Once identified, the defects can then be repaired by a repair system, or a choice can be made to reject the device, leading to manufacturing cost savings in both cases. Other applications include inspection and identification of artifact-like features in research specimens, e.g., in biology.
One particularly important use of IR thermography is the testing of the active layer, or “active plate,” of liquid-crystal display (LCD) panels. Defect analysis can be used to improve processing and increase manufacturing yield. Also important, defective panels can be repaired, provided the number and extent of defects are not too great, again increasing manufacturing yield.
FIG. 1 (prior art) depicts portions of an active plate 100 for use with an LCD panel. (FIG. 1 was taken from U.S. Pat. No. 6,111,424 to Bosacchi, which issued Aug. 29, 2000, and is incorporated herein by reference.) Active plate 100 includes a first shorting bar 105 connected to each pixel in an array of pixels 110 via a collection of source lines 115 and a second shorting bar 120 connected to each pixel 110 via a collection of gate lines (control lines) 125.
According to Bosacchi, active plate 100 is tested by evaluating the IR emission of active plate 100 with voltage applied to shorting bars 105 and 120. With power thus applied, portions of plate 100 operate as resistive circuits, and consequently dissipate heat. The heating response characteristics of plate 100 are then evaluated, preferably after plate 100 reaches a stable operating temperature (thermal equilibrium).
In the absence of defects, the pixel array should heat up uniformly. Non-uniform thermal characteristics, identified as aberrant IR intensity values, therefore indicate the presence of defects. Reference intensity values can be obtained by averaging the pixel intensity values of a given image frame, or by means of a reference frame corresponding to an ideal or defect-free reference plate.
FIG. 2 (prior art) details a portion of a conventional pixel 110, and is used here to illustrate a number of potential defects. The depicted features of pixel 110 are associated with the active plate of a liquid-crystal display, and include a thin-film transistor 200 having a first current-handling terminal connected to one of source lines 115, a control terminal connected to one of gate lines 125, and a second current-handling terminal connected to a capacitor 210. The second electrode of capacitor 210 connects to a common line 212. Pixel 110 also includes a second capacitor 211 having a liquid-crystal dielectric.
The defects, which are illustrative and not exhaustive, include both shorts and opens. The shorts are between: source line 115 and gate line 125 (short 215) or common line 212 (short 216); the two current-handling terminals of transistor 200 (short 220); the gate and second current-handling terminal of transistor 200 (short 225); and the two terminals of capacitor 210 (short 226). The opens segment the source, gate, and common lines (opens 227, 228, and 229), and are between: source line 115 and transistor 200 (open 230), gate line 125 and the control terminal of transistor 200 (open 232), capacitor 210 and common line 212 (open 235), and transistor 200 and capacitor 210 (open 233).
Each defect of FIG. 2, plus a number of others, adversely impacts the operation of pixel 110. Unfortunately, many of these defects are difficult to discover using conventional test methods. There is therefore a need for improved methods and systems for identifying and locating defects.
Some inspection systems include an excitation source that excites the object under test in a way that highlights defects to an imaging system. The type of excitation depends upon the imaging system, which may acquire images based on visible light, infrared, combined spectroscopy, magnetic fields, etc. Whatever imaging system is employed, test images of the object under test are contrasted with some reference image to obtain a composite image: significant differences between the test and reference images show up in the composite image, and identify potential defects.
Some forms of excitation produce defect artifacts, which are differences between test and reference images that are caused by defects but that do not physically correlate to defect areas. A short between two lines increases current through those lines, and consequently elevates the temperatures of the lines along with the short. Thus the lines, though not themselves defective, nevertheless appear with the short in the composite image. The defect data representing the short is thus imbedded within defect-artifact data (i.e., a “defect artifact”). Defect artifacts often obscure the associated defects, rendering them difficult to precisely locate. Human operators can locate a defect within a defect artifact by careful study under a microscope, but people are relatively slow and are quickly fatigued. There is therefore a need for means of automatically distinguishing defects from their related artifacts.